Introduction
1.1 Importance and uses of nanoparticles
“NANOPARTICLES” are the particles having dimension between 1 to 100 nm or particles of any shape with dimensions 1 to 100nm. The shape of these materials can be 1D, 2D or 3D. Nanoparticles are small pieces of a substance. These particles do not behave as a homogeneous group of substances.
The Greek word “nanos” was used to derive the word “NANO”, which means extremely small. The most fundamental component of nanostructure is a nanoparticle. Nanoparticles are incredibly useful. These are used in manufacture of sunscreens (titanium oxide), scratchproof eyeglasses (antimony-tin oxide), stain-repellent fabrics, crack-resistant paints, and ceramic coatings for solar cells.[3]
Nanotechnology is the group of procedures which can be used for manipulating matter in the dimension range of 1 to 100 nm in order to build nanosized species for useful purposes. Nanotechnology in medicine also play a very important role. Most of applications of nanoparticles in medicine are under development. The nanosponges are nanoparticles which behave as polymer and are coated with a red blood cell membrane. The red blood cells permit the nanosponges to travel with ease in the bloodstream and attract the toxins. Metallic nanoparticles possess different chemical properties from bulk metals. The optical property is one of the fundamental characteristics of a nanoparticle.
1.2 Cu2ZnSnSe4 nanoparticles
Now a days, many types of nanoparticles are synthesized by using different type of methods. Cu2ZnSnSe4 (CZTSe) nanoparticles are one of them. During synthesis, the metal salts influence the chemical composition of nanoparticles. These are semiconductor nanoparticles. Substantial changes in physical and chemical properties of semiconductor materials are observed when scaled down to nanoscale. The use of semiconductor nanoparticles over metal oxide nanoparticles is in great interest in photovoltaic applications. The metal oxide nanoparticle SiO2 is most efficiently used in solar cells, but it is much expensive as compared to CZTSe nanoparticles. We cannot use it in day-to-day life. CZTSe is a low-cost material and well accommodated for photovoltaic applications due to their band gaps (1.0-1.5eV). The absorption coefficient is also high (105 cm-1) [4,9]. CZTSe nanoparticles are more dispersed, more decrease in size, more homogeneously distributed and properties can be more stable. The morphology, composition, crystalline structure of nanoparticles was identified using many characterization techniques. [8]
2.GENERAL METHODS OF SYNTHESIS OF NANOPARTICLES
In general, various methods can be used for synthesis of nanoparticles. Synthesis methods of nanoparticles are broadly divided into two main categories-
1. Bottom-up approach- Smaller to larger particle
2. Top-down approach- Larger to smaller particle
Fig-1 Artificial methods for synthesis of nanoparticles for the (a) top-down and (b) bottom-up approaches.[2]
2.1 Top-down approach –Top-down approach means top(larger) to down(smaller). Starting from large material, the material was disintegrated into smaller units. These units were changed into suitable nanoparticles. The decomposition techniques used are- grinding, milling etc.
2.2 Bottom-up approach – Bottom-up approach means synthesis of smaller particles from large particles. This approach is also called building up approach. This approach is exactly opposite to top-down approach. This approach is used for nanoscale production because it focuses on the interconnections of atoms and molecules and their design into larger structures.
Fig-2 (A)Bottom-up approach. (B)Top-down approach.[2]
This approach is chemical synthesis approach. In this two different methods are present:-
1. Solution phase method
2. Solid phase method
Solution phase method is advantageous over solid phase method, reaction can vary very clearly between precursor and solvent and rate of a reaction is more in this method so, homogeneously we can react the reactant to get batter product. Synthetic methods which are solution based are the main approaches for nanoparticle synthesis because they have highly mobile reagents. The two stages of crystallization from solution are nucleation and growth.
The main objective of solution phase synthesis is to synthesize simultaneously large number of stable nuclei that experience little further growth.
The disadvantage of this method is that the particle can undergo Ostwald ripening, in which smaller particles redissolve and their solvated species reprecipitate on large particles, thus expanding the size distribution and diminishing the total particle count. To prevent this unwanted ripening, stabilizers are added to sustain the particles.
2.3 Morphology control
Now if we want to synthesize the nanoparticles, we must control the morphology of nanoparticles. There are two approaches-
1. Surfactant assisted/ capping agent
2. Preparation of nanoparticles without surfactant
Surfactant binds with specific surface, along that direction rate of crystal growth can be controlled. Depending upon the properties of surface specific or surface selective surfactant it can bind with specific surface. Surfaces provide energy, depending upon number of surface defects and atomic distribution surface energy of crystal plane will be different.
2.4 Main parameters for controlling the morphology of nanoparticles: –
1. Surfactant to precursor ratio
2. Concentration of surfactant and precursor
3. Concentration of solvent
4. Reaction time
5. Temperature
6. Pressure
Surfactants are using in different types of nanoparticles synthesis methods such as:-
- Colloidal Method
- Solvothermal / Hydrothermal Method
For the synthesis of nanoparticles, we have to mainly focus on stoichiometric ratio. According to stoichiometric ratio we must take precursor. Solvent can be polar or non-polar. Based on solubility of reactant, we can take solvent, mainly we use polar solvent. All are thermolysis method under surfactant assisted method. Surfactant is replaced by caping agent in colloidal synthesis method.
3. SYNTHESIS OF Cu2ZnSnSe4 NANOPARTICLES
Methods for synthesis of Cu2ZnSnSe4(CZTSe) nanoparticles-
3.1 Colloidal method-
Colloidal methods are well accomplished and simple precipitation processes, in which solutions of various ions are mixed under high(controlled) temperature and pressure resulting in formation of insoluble precipitates.
For the synthesis of CZTSe nanoparticles the stoichiometric ratio of all materials was checked and in a round bottom flask CuCl2.2H2O, ZnCl2, SnCl2.2H2O, Se powder, TOPO and oleylamine were taken and connected to vacuum gas manifold and nitrogen supplied. At some high temperature the mixture was degassed with constant shaking for some time. Later, it was purified with nitrogen and the temperature was increased upto some range. The mixture was stirred vigorously for few hours. The heating apparatus was detached on completion the reaction and the flask were kept cooling down naturally.[3]
Anhydrous toluene was added to it. The final product could be stored in air after re-dispersing it. [3] The obtained product was used further after drying in oven at temperature range for some time.
Fig.-3 Schematic of formation of various shape of CZTSe nanoparticles using different solvents.[6]
3.2 Solvothermal method –
There is no specific difference between solvothermal and hydrothermal method. In hydrothermal method solvent used is water and in solvothermal method for solvent any solvent like, ethylene glycol and ethylenediamine can be used. This method or process involves the use of a solvent under moderate to high temperature (100⁰C to 1000⁰C) and pressure (1atm to 10,000atm) which leads to interaction between precursor(s) and solvent (in a close system).
Solvothermal process was found to be useful for the synthesis of many chalcogenide compounds [1, 17]. For the synthesis of CZTSe nanoparticles by this method a teflon-lined stainless-steel autoclave was used. [18] In ethylene glycol (EG) elemental Se powder was dissolved with the help of magnetic stirring for some hours. In this mixture, copper (II) chloride dehydrates, zinc (II) chloride, tin (IV) chloride pentahydrate and polyvinaylpyrrolidone were added. [19] The mixture was stirred continuously for few minutes and was transferred into the autoclave (heating substance above their boiling point). [20] The autoclave or heating substance was closed and maintained at some temperature. Upon completion of the reaction, the autoclave was cooled naturally. The precipitates were collected after removing by-products. The final product was collected after vacuum-drying. [21,22]
3.3 Hydrothermal method– There is only a significant difference between hydrothermal method and solvothermal method. In hydrothermal method the solvent used is water (solvent above boiling point, super critical water).
Hydrothermal method was used to synthesize surfactant – free Cu2ZnSnSe4 nanocrystals in aqueous medium. Stoichiometric amount of all the salts used was mainly focused. Copper chloride dihydrate, zinc chloride and stannous chloride dihydrate were added in distilled water and stirred for some time. Disodium selenosulfate was added and pale-yellow colour appeared, and hydrazine hydrate was added to reduce the colour. The mixture was transferred in hydrothermal flask and heated. After some time, black product was collected.
The morphology of the synthesized sample was aggregated nanoparticles. [8] The surface of CZTSe nanoparticles play a very important role in evading the development of bigger particles.
4.CHARACTERIZATION OF Cu2ZnSnSe4 NANOPARTICLESS
4.1 Characterizations-
Morphological characterization– The word morphology includes shape and size of a particle. Morphology affects characteristics of nanoparticles. There are many techniques that can be used to study the morphology such as TEM, SEM and polarized optical microscopy (POM).
Structural characterization– The composition and nature of bonding materials can be studied with the help of structural characteristics. XRD, XPS, IR, Raman are the well-known techniques that are used to study the structural characteristics of nanoparticles. XRD is the most important technique to characterize.
Optical characterization– The optical properties of nanoparticles have great concerned in photo catalytic applications. These techniques provide information about the properties of absorption, reflectance, and luminescence of nanoparticles.
4.2 Characterization techniques–
As-synthesized nanoparticles can be characterized using these techniques-
- X-Ray Diffraction (XRD)
- Transmission electron microscopy (TEM)
- High resolution transmission electron microscopy (HRTEM)
- Raman spectroscopy
- FT-IR(Infrared) spectroscopy
- Field emission scanning electron microscopy (FESEM)
X-Ray diffraction– X-ray diffraction pattern of as- synthesized nanoparticles was used to determine the structure of any particle. This is one of the best characterization techniques to get the structural information about nanoparticles. The crystallite size of CZTSe nanoparticles can be determined by using Scherrer’s formula [9]
D(XRD)= 𝑲𝝀𝜟(𝟐𝜽)𝒄𝒐𝒔𝜽
Raman spectroscopy- It is aspectroscopic technique which is used to identify the rotational and vibrational frequency modes in a system. It is used to provide structural information which can be used to identify molecule. Its drawback is low sensitivity.
Transmission electron microscopy (TEM)– Through this technique, imaging of internal structure of solids can be done with the help of beam of high energy electrons transmitted through solid. [3].
5. PROPERTIES OF NANOPARTICLES
The nanoparticles are unique because of their properties like large surface area, optical activity, and chemical activity.
They possess electrical and optical properties which can be efficiently utilised.
Fig.-4 Electronic band theory.
6.APPLICATIONS-
Considering the unique properties, CZTSe nanoparticles can be used in variety of applications such as, photovoltaic, thermoelectric, and photoelectric applications.
6.1 Photovoltaic– This term deals with the conversion of light into electricity with help of semiconducting materials exhibiting photovoltaic effect.
SiO2 which is most efficient in photovoltaic application. By using semiconducting or CZTSe nanoparticles we can replace silicon dioxide. Material of silicon dioxide is very expensive. We cannot use it in day-to-day life. Material of CZTSe nanoparticles is cheaper as compared to silicon dioxide. Thus, semiconducting nanoparticles play a very important role in photovoltaic application. CZTSe based solar cells have potential to be low-cost solar cells due to natural abundance and low toxicity of the constituent elements. Photovoltaic solar cells convert the radiant light from sun directly into electricity.
Fig.-5 Schematic of copper-tin based semiconductor system and their important application [12].
6.2 Photo catalytic– The photo catalytic application of CZTSe nanoparticles was studied by using dye degradation demonstration. In dye degradation, photo catalytic reaction takes place, in which reaction absorb photon energy and electrons are excited from ground state to excited forming a hole in ground state. Excited electrons are more active and form hydroxyl free radicals and superoxide radical anions by reaction with molecular oxygen or environmental oxygen. After degradation it form water soluble species or CO2.
7.CONCLUSION
In conclusion, we reported a detailed overview about synthesis and characterization of CZTSe nanoparticles, properties, and their applications. The use of nanoparticles incredibly increased in day-to-day life. Synthesis of CZTSe nanoparticles was done by using different methods. The bottom-up approach is mainly used for synthesis of nanoparticles. Synthesized nanoparticles by colloidal method are easily dispersible in non-polar solvents. By using surfactants morphology was also controlled. Due to tiny size, nanoparticles have large surface area. The as – synthesized materials are characterized by using XRD, TEM, EDS, FESEM and SAED techniques. Structural properties are easily informed my XRD. Optical characterization was used to know about the band gap of particles. CZTSe are semiconductors and play important role in photovoltaic application. CZTSe material is cheap and thus used in solar cells.
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